Although dip coating is widely used and is the preferred method for manufacturing photoconductor drums, not much has been published on the subject. A review paper by M. Aizawa in Denshi Shashin Gakkai-shi (Electrophotography), Vol. 2, No. 9, pp. 54-63 (186-195) reports that the formation of the coating film is influenced by the coating environment (temperature, humidity, and cleanliness) as well as by removal of bubbles from the coating solution, turbulence of the coating process, homogeneity of the drum surface (interfacial tension between surface and coating liquid), and other factors.
A critical issue in dip coating for the manufacture of photoconductor drums is the control of both thickness and thickness uniformity, especially for high quality printing. U.S. Pat. No. 4,618,559, the disclosure of which is incorporated herein by reference, describes the impact of this problem on the uniformity of photosensitivity of the coated drum. Thickness non-uniformity of the charge transport layer results in non-uniform photosensitivity. To deal with the problem, the reference describes an improved process for preparing an electrophotographic photosensitive member having a charge generation layer on a substrate and a charge transport layer, each formed by dip-coating, wherein the charge transport layer forms a predetermined irregular end film portion of length H that impairs the photosensitivity of the member. The improvement in the process entails controlling the thickness of the charge generation layer over the end film portion H by varying the withdrawal rate of the substrate during the dip coating of the charge generation layer in accordance with a specified formula.
U.S. Pat. No. 6,270,850, the disclosure of which is incorporated herein by reference, describes a method for improving the quality of a dip coated layer that is deposited by flowing a solution along a substrate in a gap between the substrate and a wall, including: (a) determining a yield stress, a viscosity, a density, and a surface tension of the solution, and selecting a wet thickness of the coated layer; (b) determining a coating speed based on the determined viscosity, the determined density, the determined surface tension of the solution, and the selected wet layer thickness; and (c) selecting a distance for the gap and calculating the shear stress of the solution in the gap based on the gap distance, wherein the shear stress is greater than the yield stress.
U.S. Pat. No. 6,270,850 discusses coating non-uniformities such as streaking, marbling and sloping, i.e., a top to bottom thickness difference on a drum and suggests that some of most of these defects are caused by non-Newtonian coating solutions that can be mitigated by selecting an appropriate gap distance between the substrate and the dip coating vessel. The limitation of this approach resides in the fact that the coating vessel itself has to be adjusted for a given coating composition and a given coating wet thickness. In a production environment, the coating vessel is expensive and fixed, which limits flexibility for coating different products. There is a need to develop a method to deal with the sloping problem in a more general way that does not require modification to the coating vessel and is economical to practice.
P. Groenveld, “Thickness Distribution in Dip-Coating,” J. Paint Technology, Vol. 43, No. 561, October 1971, the disclosure of which is incorporated herein by reference, discusses the varying thickness of a film on a vertical, flat plate being withdrawn from a bath of paint. FIG. 1 depicts the thickness distribution in dip coating of a theoretical endless plate compared with a plate of finite length. For the latter situation, draining of the dipped plate upon removal from the dip tank results in an, uneven parabolic thickness distribution of at least a portion of the plate. If no solidification of the coating occurs, the entire film will be of uneven thickness. In most situations however, the paint film solidifies during the withdrawal, for example, through evaporation. In that case, a distribution containing a portion of uniform thickness is obtained, as shown in the FIGURE.
The equation describing the Groenveld model is very complicated. However by analyzing the results supporting the model, the present inventor has deduced that the most important parameters controlling the dip coating process are the following: coating solution viscosity, coating substrate withdrawal speed, coating solution surface tension, and evaporation rate of the coating solution solvent. Three of these parameters can be combined, as shown in Equation 1 below, to yield a dimensionless capillary number Ca:Ca=(mv)/S   (Equation 1)where v is the substrate withdrawal velocity in cm/sec, m, the dynamic viscosity of the coating solution in poise, and S the surface tension of the solution in dyne/cm.
Using an existing coating apparatus in a normal coating environment to carry out a series of tests that include variation of the aforementioned four key parameters (reduced to two when the capillary number is used), the present inventor has discovered a model that defines the coating process and enables control of the sloping problem.